This invention relates to pressurisation pumps. In particular, this invention also relates to fuel pumps and especially, but not exclusively, to fuel pumps used with compression ignition internal combustion engines.
In all types of internal combustion engines it is important for fuel economy that as much of, if not all of, the fuel injected into a combustion chamber is consumed during each combustion cycle. As a first step towards that goal it is important that fuel injected into the combustion chamber is atomised as much as possible as this helps the combustion process by increasing the available fuel surface area for oxidation. Another important consideration is to ensure that the fuel is spread as homogeneously as possible throughout the combustion chamber as this aids flame propagation and so improves combustion efficiency. It follows then that for efficient operation the fuel needs to be injected as fast as possible to provide time to diffuse sufficiently before ignition and the fuel also needs to be injected under as high pressure as possible to ensure maximum atomisation. These factors are especially important for compression ignition engines, or diesel engines, as they rely on compressing air in the combustion chamber to high enough pressures so that the accompanying increase in temperature is hot enough to ignite diesel fuel injected into the combustion chamber, without using premixing or other techniques used in modern petrol engines to aid efficient combustion.
It is also important that the injection phase of each combustion cycle is controlled as tightly as possible to allow accurate fuel metering and ensure that the correct amount of fuel is injected to match engine load requirements, In known diesel engine systems fuel travels from a fuel pump to each individual cylinder of the engine in separate pipes. Fuel injection in the past has been handled by cam-driven injection systems, such as inline pumps, distributor pumps, unit injectors and unit pumps. These systems build up fuel injection pressure for each injection of fuel and are powered by the engine. Fuel metering and pressure build-up are therefore linked and cannot be separated. The injection pressure results from the metered fuel quantity being pushed through the injector nozzle orifice by an injection piston contained in the injector, and as the injection piston velocity is proportional to engine speed, so the resultant fuel pressure is also proportional to engine speed.
This link between engine speed and injection pressure in previous systems meant that only limited pressure is available at low engine speeds, harming fuel economy and delivering sluggish responsiveness, slow acceleration and a perception of unrefinement to the diesel automobile operator. In addition, in engines running at high speed there is reduced time on offer, compared to engines running at low speeds, for the air and fuel to mix sufficiently to allow complete combustion. It is clear that injection pressure is key to moving the combustion process along at the fast pace demanded by high-speed engines, and decoupling pressure generation from injection is also highly desired for the reasons explained above.
It was to address the above problems that common-rail diesel systems were developed. A typical common-rail system comprises a fuel supply pump, a common rail (or accumulator) and injectors all joined by high-pressure piping, an electronic control unit, and electronic driver unit and various sensors. The supply pump maintains high fuel pressure inside the rail and fuel is injected by opening and closing an internal electromagnetic valve in each injector. Hence, there is no relationship between engine speed and injection pressure. The common-rail system enables fuel to be injected into the engine's combustion chambers at very high pressures, so the fuel and air mix more thoroughly and burn more efficiently than previous systems. Additionally, as the fuel pump constantly replenishes the common rail with pressurised fuel, high pressure is maintained throughout the engine's range of speeds, thus solving the problem of hesitation on acceleration and improving refinement.
More recent inventions relating to common rail systems have been those of providing additional pressurisation in the unit injectors and direct fuel injection. However, with all these systems the common goal is to improve fuel economy, reduce emissions, reduce complexity and reduce the weight of the engine and dependent ancillaries.
Previous fuel pumps used with inline, distributor and common-rail systems used mechanically actuated valves to control input and output fuel from the fuel pump. Mechanical valving inevitably introduces losses through expenditure of energy in opening and closing inlet and outlet valves. Additionally, as these fuel pumps tend to be driven by the engine, utilising engine power to operate mechanical valving systems draws torque from the engine, resulting in less torque being available for useful work and hence, a further reduction in engine efficiency. Known fuel pumps have also tended to be complex and costly units—consequently there is a desire to reduce complexity as it brings obvious attendant advantages to both the manufacturer and consumer in terms of cost and reliability. With increasingly stringent emissions demands placed upon automobile manufacturers weight is also an important issue as weight has a direct effect on fuel consumption. All the desired improvements mentioned above are synergistic to improving efficiency. Furthermore, as the fuel pump in common-rail applications is running at high speed at all times during engine operation, even a small improvement in efficiency will produce appreciable gains over the long term.